In the case of aeroplane turbojets, it is crucial to determine precisely the mass flow of fuel being injected into the combustion chamber of the turbojet. This is because by estimating the mass flow the mass flow of reduced fuel can be calculated, as well as the richness of the air/kerosene mix. The more precise this estimate of mass flow, the more precise the calculation of mass flow of reduced fuel as well as the richness of the air-kerosene mix. Since the acceleration and deceleration limits are generally expressed as a function of the mass flow of reduced fuel, the lack of accuracy in calculating this flow reduces the performance of the engine. Moreover, the control of polluting emissions requires knowledge of the richness of the air/kerosene mix of the chamber, and the lack of precision in calculating this richness reduces the quality of this control. In general, control of the engine is made possible by estimating the mass flow of fuel injected.
It is currently known to estimate the mass flow of fuel injected into the combustion chamber in two different ways. The mass flow of fuel injected may be measured from the measurement of the position of the fuel metering device and from an associated equation. However, since the measurement is a random signal, the mass flow calculated is subject to interference. Moreover, the equation linking the position of the metering device to the mass flow passing through the same contains some unknowns in its parameters. These unknowns are due in particular to the variability of the density and viscosity of the fuel, to manufacturing tolerances in the geometric features of the metering device, to the precision of hydromechanical control of the pressure differential through the metering device and to the precision of electromechanical control of the position of the slide valve of the metering device.
The mass flow of fuel injected may also be measured from the measurement of mass flow by a conventional flow meter. However, the dynamics of a mass flow meter are too poor for use in the control of an engine.